[linrad] Re: Noise Reduction

["sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

Hi Rein,

> So far I have always used linrad and winrad with output from a 
> ssb receiver ( 1.8 kHZ filter ) not realizing that this might 
> not be the optimum conditions for the noise reducer and noise 
> blanking to perform..

It depends. The Linrad blanker may work perfectly well if the
noise pulses are separated by more than 0.5 ms or so. (In contrast
to simple blankers that do not compensate for the phase response
of the SSB filter.)

The Linrad calibration procedure sets up a filter that corrects
for sub-optimal amplitude/phase response of an analog receiver.
Some pretty old info is available here:
http://www.sm5bsz.com/pcdsp/blnini.htm

With Linrad one can set the sampling speed at just about twice 
the upper audio frequency. For 300-2100 Hz, sample at 5kHz
(if your soundcard can go that low) The calibration routine
will make noise pulses extend one or two 2 samples, typically
5 times shorter than in the original audio.
(see http://www.sm5bsz.com/linuxdsp/pulresp/pulresp.htm )
Energy is conserved so the pulses also become much higher 
and easier to locate. Linrad will subtract the weak 
oscillations that belong to the short pulse. This is a 
linear operation that does not introduce intermodulation 
or loss of information.

The process works ONLY if the radio has adequate dynamic range
within the passband. Many receivers do not, AGC must be off
and RF volume low enough for the interference pulses to stay 
within the linear range.

More bandwidth is absolutely necessary when pulses have higher
repetition frequencies. If the pulses can not be separated 
from each other, the process fails.

With a bandwidth of 5 MHz on an analog system it is possible 
to remove static rain almost completely even though it
sounds like white noise and lifts the S-meter to S9+20 with
the blanker switched off.
A description in Swedish is linked to from this page:
http://www.sm5bsz.com/blanker.htm

I would think that a bandwidth in the order of 1 MHz would be 
enough for Linrad to remove static rain.

73

Leif


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